In base stations for personal communication systems (GSM, EDGE, W-CDMA), RF-power amplifiers (PA) are key components. For these power amplifiers, RF Laterally Diffused MOS (LDMOS) transistors are the standard choice of technology, because of their excellent power capabilities gain, linearity and reliability.
An RF power amplifier is a type of electronic amplifier used to convert a low-power radio-frequency signal into a larger signal of significant power, typically for driving the antenna of a transmitter. It is usually optimized to have high efficiency, high output Power (P1dB) compression, good return loss on the input and output, good gain, and optimum heat dissipation. The basic applications of the RF power amplifier include driving another high power source, driving a transmitting antenna, microwave heating, and exciting resonant cavity structures. Among these applications, driving transmitter antennas is most well-known. The transmitter-receivers are used not only for voice and data communication but also for weather sensing (in the form of radar).
An RF power amplifier generally comprises of a package having a source lead (plate shaped), a drain lead, and a gate lead. Inside the package there is provided a semiconductor die in which the RF power transistor is manufactured. Depending on the size of the semiconductor die, there may also be a capacitor (MOSCAP) provided at the gate side of the die and there may be a further capacitor at the drain side. The capacitor(s) are added for impedance matching of the semiconductor die to the outside world. The die is wire bonded to the respective leads. For (electrical) performance reasons, the bond wires are kept as short as possible. Power amplifiers (PA's) for base stations use predominantly laterally-diffused metal-oxide-semiconductor (LDMOS) technology. The trend in base station PA's is towards higher peak power capabilities to be able to transmit more channels for larger data capacity. Modern LDMOS dies are designed in such a way that the gate and drain contacts are at the topside of the die and the source contact is on the backside of the die. The LDMOS packages are designed to fit this configuration. The gate and drain can be connected to the leads through bond wires. The source is connected to the bottom (lead) of the package (flange) using the backside contact of the die. The flange acts as a third lead. This configuration ensures a very short connection between the die and the third lead (source). This back-side contacting of the source is only possible in case the substrate of the die is conducting, which is the case in silicon-based LDMOS technology.
Another important performance indicator for power RF-LDMOS transistors is the RF-ruggedness. This RF-ruggedness may be defined as the ability to withstand reflected power at the output. Power is reflected at the output if an impedance mismatch occurs. This may occurs as an incident (e.g. at breaking of the antenna) or as a structural, normal part of the application (e.g. at switching on a lamp). The amount of reflected power a device is able to withstand is expressed in the voltage standing wave ratio (VSWR) this device is able survive (without breaking down). In many LDMOS products for base stations, a VSWR of 1:10 is specified. In an RF power amplifier the reflected power causes voltage peaks at the drain of the LDMOS in the amplifier stage. If this voltage becomes too high, the device is destroyed.
In the prior art several solutions to this problem have been proposed.
A first solution concerns the protection of a power amplifier (PA) for VSWR-mismatch by using a circulator (http://en.wikipedia.org/wiki/Circulator) at the output of the device. This is a very expensive solution.
Another solution is reported in U.S. Pat. No. 4,122,400. This published patent discloses a protection circuit for a transmitter amplifier, in which separate VSWR (reflected power) control and separate temperature control is provided when the thresholds of either or both are exceeded. When the temperature of the RF amplifier is sensed and when this temperature exceeds a certain threshold level, the output level of the RF amplifier is adjusted to protect the amplifier. Separately, there is provided a means for sensing the ration of the reflected power to the forward power (by an expensive directional couple) and when this ration exceeds a given threshold, the gain of the amplifier is cut back to thereby protect the amplifier.
A further solution is reported in U.S. Pat. No. 6,794,719B2 in which an integrated diode is proposed for ruggedness improvement of a high-voltage (HV) LDMOS transistor. However, this solution is not suitable for RF-applications. In this solution the diode must cope with a high current and high voltage (breakdown voltage of the diode), which gives large power dissipation. To avoid thermal damage a large diode must be used, which gives a large capacitance, and this gives a decrease in RF-performance.
Further ruggedness can obtained increasing the intrinsic ruggedness of the LDMOS itself. This is done by avoiding the turn-on of the bipolar NPN-transistor, which is inherently present in an LDMOS transistor. If this parasitic bipolar transistor is switching on, the device is destroyed. Avoiding turning-on of the bipolar transistor can be done by electrical field engineering, and/or lowering the base resistance of the bipolar. This is also described in publication of S. J. C. H. Theeuwen et al. al., “LDMOS Ruggedness reliability”, Microwave Journal, Vol. 52/No. 4/April 2009/p. 96-104. However, there will always remain a trade-off between RF-performance versus intrinsic ruggedness. Furthermore, for future development of LDMOS on high resistivity substrate, it will become very hard to introduce sufficient intrinsic ruggedness into the power RF-LDMOS.